Conversion of relic gravitational waves into photons in cosmological magnetic fields

TL;DR

This paper investigates how relic gravitational waves can convert into X-ray photons in cosmological magnetic fields, potentially contributing significantly to the cosmic X-ray background, especially if primordial black holes dominated early universe conditions.

Contribution

It provides a detailed calculation of graviton-to-photon conversion probabilities during different cosmological epochs, highlighting a novel mechanism for generating observable electromagnetic radiation.

Findings

Relic gravitons can convert into X-ray photons in cosmological magnetic fields.

Conversion probability is significant during the recombination epoch and later stages.

Converted electromagnetic radiation could dominate the cosmic X-ray background if PBHs were prevalent.

Abstract

Conversion of gravitational waves into electromagnetic radiation is discussed. The probability of transformations of gravitons into photons in presence of cosmological background magnetic field is calculated at the recombination epoch and during subsequent cosmological stages. The produced electromagnetic radiation is concentrated in the X-ray part of the spectrum. It is shown that if the early Universe was dominated by primordial black holes (PBHs) prior to Big Bang Nucleosynthesis (BBN), the relic gravitons emitted by PBHs would transform to an almost isotropic background of electromagnetic radiation due to conversion of gravitons into photons in cosmological magnetic fields. Such extragalactic radiation could be noticeable or even dominant component of Cosmic X-ray Background.